One-pot synthesis of cyclic nitrones and their conversion to pyrrolizidines: 7a-epi-crotanecine inhibits α-mannosidases

Article abstract of DOI:10.1021/jo0523518

A new straightforward and inexpensive one-pot procedure is described for the preparation of enantiopure five-membered cyclic nitrones starting from the corresponding lactols. Its efficiency relies on the condensation of unprotected hydroxylamine with read

Full text of DOI:10.1021/jo0523518

One-Pot Synthesis of Cyclic Nitrones and Their Conversion to
Pyrrolizidines: 7a-epi-Crotanecine Inhibits r-Mannosidases
S. Cicchi,† M. Marradi,† P. Vogel,‡ and A. Goti*,†
Dipartimento di Chimica Organica “Ugo Schiff”, UniVersita` di Firenze, ICCOM C.N.R.,
Via della Lastruccia 13, I-50019 Sesto Fiorentino (FI), Italy, and Laboratory of Glycochemistry and
Asymmetric Synthesis (LGSA), Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), BCH,
CH-1015 Lausanne, Switzerland
andrea.goti@unifi.it
ReceiVed NoVember 14, 2005
A new straightforward and inexpensive one-pot procedure is described for the preparation of enantiopure
five-membered cyclic nitrones starting from the corresponding lactols. Its efficiency relies on the
condensation of unprotected hydroxylamine with readily available lactols and on the chemoselectivity of
the subsequent esterification with methanesulfonyl chloride. The targeted enantiomerically pure pyrroline
N-oxides are versatile synthetic intermediates: one of the nitrones so-obtained has been converted into
new polyhydroxypyrrolizidines, analogues of the alkaloids rosmarinecine and crotanecine, which were
assayed for their inhibitory activities toward 22 commercially available glycosidase enzymes. One of
them ((-)-7a-epi-crotanecine) is a potent and selective inhibitor of R-mannosidases from jack beans and
almonds.
Introduction
pling reactions.3 Enantiomerically pure and polyfunctional cyclic
nitrones such as 14 and 2,5 derived from malic and tartaric acid,
respectively, have found applications in the total, asymmetric
synthesis of polyhydroxylated pyrrolidine, indolizidine, and
pyrrolizidine alkaloids.6 The latter compounds have shown
interesting properties as glycosidase inhibitors7 and as such are
potential therapeutic agents.8 Several glycosidase inhibitors are
being currently tested or approved in the treatment of diabetes,9
Nitrones are useful synthetic intermediates as they undergo
several synthetically useful reactions such as 1,3-dipolar cy-
cloadditions,1 nucleophilic additions,1a,2 and pinacol-type cou-
* To whom correspondence should be addressed. Phone: +39-0554573505.
Fax: +39-0554573531.
† Universita` di Firenze.
‡ Ecole Polytechnique Fe´de´rale de Lausanne.
(1) (a) Merino, P. In Science of Synthesis; Padwa, A., Ed.; Thieme:
Stuttgart, Germany, 2004; Vol. 27, pp 511-580. (b) Tufariello, J. J. In
1,3-Dipolar Cycloaddition Chemistry; Padwa, A., Ed.; John Wiley &
Sons: New York, 1984. (c) Confalone, P. N.; Huie, E. M. Org. React.
1988, 36, 1-173. (d) Torssell, K. B. G. In Nitrile Oxides, Nitrones, and
Nitronates in Organic Synthesis; Feuer, H., Ed.; VCH Publishers: New
York, 1988. (e) Jones, R. C. F.; Martin, J. N. The Chemistry of Heterocyclic
Compounds. In Synthetic Applications of 1,3-Dipolar Cycloaddition
Chemistry Toward Heterocycles and Natural Products; Padwa, A., Pearson,
W. H., Eds.; John Wiley & Sons: New York, 2002; Vol. 59, pp 1-81. (f)
Desimoni, G.; Tacconi, G.; Barco, A.; Pollini, G. P. Natural Products
Synthesis Through Pericyclic Reactions, ACS Monograph No. 180; Caserio,
M. C., Ed.; American Chemical Society: Washington, DC, 1983. (g)
Annunziata, R.; Cinquini, M.; Cozzi, F.; Raimondi, L. Gazz. Chim. Ital.
1989, 119, 253-270. (h) Frederickson, M. Tetrahedron 1997, 53, 403-
425. (i) Gothelf, K. V.; Jørgensen, K. A. Chem. ReV. 1998, 98, 863-909.
(j) Cardona, F.; Goti, A.; Brandi, A. Eur. J. Org. Chem. 2001, 2999-3011.
(k) Osborn, H. M. I.; Gemmell, N.; Harwood, L. M. J. Chem. Soc., Perkin
Trans. 1 2002, 2419-2438. (l) Koumbis, A. E.; Gallos, J. K. Curr. Org.
Chem. 2003, 7, 585-628.
(2) (a) Lombardo, M.; Trombini, C. Synthesis 2000, 759-774. (b)
Lombardo, M.; Trombini, C. Curr. Org. Chem. 2002, 6, 695-713. (c)
Merino, P.; Merchan, F. L.; Franco, S.; Tejero, T. Synlett 2000, 442-454.
(3) For a brief account, see: (a) Cardona, F.; Goti, A. Angew. Chem.,
Int. Ed. 2005, 44, 7832-7835. For fundamental contributions, see: (b)
Masson, G.; Cividino, P.; Py, S.; Valle´e, Y. Angew. Chem., Int. Ed. 2003,
42, 2265-2268. (c) Masson, G.; Py, S.; Valle´e, Y. Angew. Chem., Int. Ed.
2002, 41, 1772-1775. (d) Riber, D.; Skrydstrup, D. Org. Lett. 2003, 5,
229-231. (e) Zhong, Y.-W.; Xu, M.-H.; Lin, G.-Q. Org. Lett. 2004, 6,
3953-3956.
(4) (a) Cicchi, S.; Goti, A.; Brandi, A. J. Org. Chem. 1995, 60, 4743-
4748. (b) Goti, A.; Cacciarini, M.; Cardona, F.; Brandi, A. Tetrahedron
Lett. 1999, 40, 2853-2856. (c) Goti, A.; Cicchi, S.; Fedi, V.; Nannelli, L.;
Brandi, A. J. Org. Chem. 1997, 62, 3119-3125. (d) Merino, P.; Tejero,
T.; Revuelta, J.; Romero, P.; Cicchi, S.; Mannucci, V.; Brandi, A.; Goti,
A. Tetrahedron: Asymmetry 2003, 14, 367-369.
(5) (a) Cicchi, S.; Ho¨ld, I.; Brandi, A. J. Org. Chem. 1993, 58, 5274-
5275. (b) Brandi, A.; Cicchi, S.; Goti, A.; Koprowski, M.; Pietrusiewicz,
K. M. J. Org. Chem. 1994, 59, 1315-1318.
10.1021/jo0523518 CCC: $33.50 © 2006 American Chemical Society
Published on Web 01/27/2006
1614
J. Org. Chem. 2006, 71, 1614-1619

One-Pot Synthesis of Cyclic Nitrones
Gaucher’s disease,10 HIV infection,11 viral infection,12 or
cancer.13 Some of them have also been used as chemical probes,
in combination with protein crystallography and kinetic studies,
to provide new insights into glycosidase mechanisms.14
crotanecine ((-)-8) has been found to be a potent and selective
inhibitor of R-mannosidases from jack beans and almonds.
Results and Discussion
Racemic nitrone (()-3 has been obtained by Wightman and
co-workers6g and by us6a through oxidation of the corresponding
meso-pyrrolidine or N-hydroxypyrrolidine, respectively, derived
from 2,3-O-isopropylidene-D-erythrose ((-)-9). Enantiomeri-
cally pure nitrone (-)-3 was obtained by Closa and Wightman15
according to a four-step process starting with (-)-9.16 On our
side, we reported that (-)-3 can be derived from (-)-9 in a
one-pot process using NH2OSiMe2(t-Bu), as shown in Scheme
1.17
We describe, herein, efficient syntheses for the known cyclic
nitrones (-)-3 and (-)-4 and for the yet unknown analogues
(()-5 and (-)-6. Cycloaddition of (-)-3 to dimethyl maleate
has led us to prepare pyrrolizidinones. One of them has been
converted into the new polyhydroxylated pyrrolizidines (-)-7
and (-)-8, analogues of rosmarinecine and crotanecine, respec-
tively. They have been tested as potential inhibitors toward 22
commercially available glycosidases. Interestingly, 7a-epi-
SCHEME 1
(6) For leading references, see: (a) Goti, A.; Cicchi, S.; Cacciarini, M.;
Cardona, F.; Fedi, V.; Brandi, A. Eur. J. Org. Chem. 2000, 3633-3645
and references therein. (b) Goti, A.; Cacciarini, M.; Cardona, F.; Cordero,
F. M.; Brandi, A. Org. Lett. 2001, 3, 1367-1369. (c) Cardona, F.; Goti,
A.; Brandi, A. Org. Lett. 2003, 5, 1475-1478. (d) Goti, A.; Cicchi, S.;
Mannucci, V.; Cardona, F.; Guarna, F.; Merino, P.; Tejero, T. Org. Lett.
2003, 5, 4235-4238. (e) Cardona, F.; Moreno, G.; Guarna, F.; Vogel, P.;
Schuetz, C.; Merino, P.; Goti, A. J. Org. Chem. 2005, 70, 6552-6555. (f)
Giovannini, R.; Marcantoni, E.; Petrini, M. J. Org. Chem. 1995, 60, 5706-
5707. (g) McCaig, A. E.; Meldrum, K. P.; Wightman, R. H. Tetrahedron
1998, 54, 9429-9446. (h) Murahashi, S.-I.; Imada, Y.; Ohtake, H. J. Org.
Chem. 1994, 59, 6170-6172.
(7) (a) Stu¨tz, A. E. Iminosugars as glycosidase inhibitors: nojirimycin
and beyond; Wiley-VCH: Weinheim, Germany, 1999. (b) Elbein, A. D.;
Molyneux, R. J. In Alkaloids: Chemical and Biological PerspectiVes;
Pelletier, S. W., Ed.; Wiley-VCH: New York, 1987; Vol. 5, Chapter 1. (c)
Legler, G. AdV. Carbohydr. Chem. Biochem. 1990, 48, 319-384. (d)
Winchester, B.; Fleet, G. W. J. Glycobiology 1992, 2, 199-210.
(8) (a) Lillelund, V. H.; Jensen, H. H.; Liang, X.; Bols, M. Chem. ReV.
2002, 102, 515-553. (b) Martin, O. R.; Compain, P. Curr. Top. Med. Chem.
2003, 3, i-iv.
In the case of the preparation of enantiomerically pure (-)-
4, we18 first applied a similar approach to 2,3,5-O-tribenzyl-L-
xylose and 2,3,5-O-tribenzyl-D-arabinose.19,20 Both routes re-
quired three steps. As we shall show in this report, much more
efficient one-pot processes have now been developed for the
(9) Mitrakou, A.; Tountas, N.; Raptis, A. E.; Bauer, R. J.; Schulz, H.;
Raptis, S. A. Diabetic Med. 1998, 15, 657-660.
(10) (a) Butters, T. D.; Dwek, R. A.; Platt, F. M. Curr. Top. Med. Chem.
2003, 3, 561-574. (b) Schueler, U. H.; Kolter, T.; Kaneski, C. R.; Zirzow,
G. C.; Sandhoff, K.; Brady, R. O. J. Inherited Metab. Dis. 2004, 27, 649-
658. (c) Lysek, R.; Schu¨tz, C.; Vogel, P. Bioorg. Med. Chem. Lett. 2005,
15, 3071-3075.
(11) (a) Groopman, J. E. ReV. Infect. Dis. 1990, 12, 908-911. (b) Varki,
A. Glycobiology 1993, 3, 97-130. (c) Robina, I.; Moreno-Vargas, A. J.;
Carmona, A. T.; Vogel, P. Curr. Drug Metab. 2004, 5, 329-361.
(12) (a) Laver, W. G.; Bischofberger, N.; Webster, R. G. Sci. Am. 1999,
280, 78-87. (b) Greimel, P.; Spreitz, J.; Stu¨tz, A. E.; Wrodnigg, T. M.
Curr. Top. Med. Chem. 2003, 3, 513-523. (c) Nash, R. Chem. World 2004,
42-44.
(13) (a) Goss, P. E.; Reid, C. L.; Bailey, D.; Dennis, J. W. Clin. Cancer
Res. 1997, 3, 1077-1086. (b) Zitzmann, N.; Mehta, A. S.; Carroue´e, S.;
Butters, T. D.; Platt, F. M.; McCauley, J.; Blumberg, B. S.; Dwek, R. A.;
Block, T. M. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 11878-11882. (c)
Fiaux, H.; Popowycz, F.; Favre, S.; Schu¨tz, C.; Vogel, P.; Gerber-Lemaire,
S.; Juillerat-Jeanneret, L. J. Med. Chem. 2005, 48, 4237-4246.
(14) (a) Heightman, T. D.; Vasella, A. T. Angew. Chem., Int. Ed. 1999,
38, 750-770. (b) Vasella, A.; Davies, G. J.; Bo¨hm, M. Curr. Opin. Chem.
Biol. 2002, 6, 619-629. (c) Yip, V. L. Y.; Withers, S. G. Org. Biomol.
Chem. 2004, 2, 2707-2713.
(15) Closa, M.; Wightman, R. H. Synth. Commun. 1998, 28, 3443-3450.
(16) See also: Tamura, O.; Toyao, A.; Ishibashi, H. Synlett 2002, 1344-
1346.
(17) Cicchi, S.; Corsi, M.; Brandi, A.; Goti, A. J. Org. Chem. 2002, 67,
1678-1681.
(18) Cardona, F.; Faggi, E.; Liguori, F.; Cacciarini, M.; Goti, A.
Tetrahedron Lett. 2003, 44, 2315-2318.
(19) Two other articles have then reported the preparation of (-)-4 using
similar procedures. From 2,3,5-O-tribenzyl-D-arabinose, see: (a) Carmona,
A. T.; Wightman, R. H.; Robina, I.; Vogel, P. HelV. Chim. Acta 2003, 86,
3066-3073. From 2,3,5-O-tribenzyl-L-xylose, see: (b) Desvergnes, S.; Py,
S.; Valle´e, Y. J. Org. Chem. 2005, 70, 1459-1462.
(20) For related studies, see also: (a) Holzapfel, C. W.; Crous, R.
Heterocycles 1998, 48, 1337-1346. (b) Duff, F. J.; Vivien, V.; Wightman,
R. H. Chem. Commun. 2000, 2127-2128. (c) Toyao, A.; Tamura, O.;
Takagi, H.; Ishibashi, H. Synlett 2003, 35-38. (d) Chevrier, C.; LeNouen,
D.; Neuburger, M.; Defoin, A.; Tarnus, C. Tetrahedron Lett. 2004, 45,
5363-5366.
J. Org. Chem, Vol. 71, No. 4, 2006 1615

Cicchi et al.
SCHEME 2
SCHEME 4
SCHEME 5
SCHEME 3
SCHEME 6
preparation of nitrones (-)-3 and (-)-4 and of the new
analogues (()-5 and (-)-6.
In an exploratory study, we found that the reaction of lactol
(-)-9 with unprotected hydroxylamine‚HCl generated the oxime
13,21 which reacted with (t-Bu)Me2SiCl preferentially on the
primary alcohol giving 15 (Z/E ) 1:8). When the crude reaction
mixture was treated with methanesulfonyl chloride, carbonitrile
16 was isolated as the main product (Scheme 2). These results
suggested that esterification of 13 with a sulfonyl chloride might
also occur preferentially at the primary alcohol moiety rather
than at the oxime moiety. Indeed, the reaction of isolated oxime
13 with MsCl afforded nitrone (-)-3, but in a poor 25% yield.
Unexpectedly, the same reaction was much more efficient when
performed in a one-pot manner. When the crude mixture
resulting from mixing (-)-9 with NH2OH‚HCl in dry pyridine
was reacted with CH3SO2Cl, (-)-3 was obtained in a gratifying
54% yield, after purification by column chromatography on
silica gel (Scheme 3). A secondary product was also isolated
and identified as nitrile 17.21 Its amount could be limited by
using only a slight excess of methanesulfonyl chloride (1.2
equiv), which was added rapidly to the reaction mixture as a
solution in dry pyridine to avoid high local concentrations of
CH3SO2Cl. The exchange of CH3SO2Cl for 2-nitrobenzene-
sulfonyl chloride and/or of pyridine for dichloromethane led to
complex reaction mixtures. Attempts using stoichiometric
amounts of bases were not met with more success.
Unfortunately, when applied to lactols 22-24, our one-pot
method gave intractable mixtures of products.
The extension of our one-pot procedure to the synthesis of
5-substituted pyrroline N-oxides was not straightforward, as it
was demonstrated in the case of oxime 26 derived from lactol
25 that its oxime moiety reacts with (t-Bu)Me2SiCl in pyridine
faster than its secondary alcohol moiety, giving selectively 27
(Scheme 5).20d
This did not impede us from running a reaction with lactol
28, derived from L-xylose with NH2OH‚HCl first and then with
CH3SO2Cl in pyridine. To our delight, after treatment of the
crude reaction mixture so-obtained with aqueous NaOH at 0
°C,24 nitrone (-)-4 was isolated in 25% yield (Scheme 6) after
column chromatography on silica gel. The same procedure
applied to the D-arabinose derivative 30 produced nitrone (-)-6
in 33% yield. Spectral data of (-)-6 confirmed that it is the
5-epimer of (-)-4.18,19
Our previous, multistep synthesis of (-)-4 from 28 gave
yields ranging from 30 to 40%.18,19 The moderate yields of the
conversions of 28 and 30 to (-)-4 and (-)-6, respectively, are
balanced by the simplicity of the one-pot procedures and by
the use of less-expensive reagents (NH2OH instead of NH2-
OSiR3). Nitrone (-)-4 is an important synthetic intermediate.
It has been used in the syntheses of pyrrolizidine alkaloids such
To extend the scope of this one-pot procedure to the synthesis
of other cyclic nitrones, we treated the racemic lactol 20,
(derived from lactone 18,22 Scheme 4) with NH2OH‚HCl and
then with MeSO2Cl in pyridine. It led to nitrone (()-5, which
was isolated in 36% yield. Its structure was deduced from its
1H and 13C NMR spectra, which showed typical low field signals
(δH 6.85 ppm, δC 135.0 ppm) for its sp2-CH moiety. It is a
close analogue of the already described nitrone (-)-21.23
(21) Buchanan, J. G.; Jigajinni, V. B.; Singh, G.; Wightman, R. H. J.
Chem. Soc., Perkin Trans. 1 1987, 2377-2384.
(22) Pattenden, G.; Reynolds, S. J. J. Chem. Soc., Perkin Trans. 1 1994,
379-385.
(23) Starting from L-aspartic acid, nitrone (-)-21 had been obtained in
a 12.6% overall yield after a much longer procedure; see ref 4c.
(24) Stempel, A.; Douvan, I.; Sternbach, L. H. J. Org. Chem. 1968, 33,
2963-2966.
1616 J. Org. Chem., Vol. 71, No. 4, 2006

One-Pot Synthesis of Cyclic Nitrones
TABLE 1. Inhibitory Activities of Pyrrolizidines (-)-7 and (-)-8a
SCHEME 7
enzyme
(-)-7‚HCl
(-)-8‚HCl
R-galactosidase from coffee beans
â-galactosidase from bovine liver
Amyloglucosidase from Rhizopus mold
â-glucosidase from almonds
NIb
NIb
NIb
NIb
24
41
32
23
33
R-mannosidase from jack beans
98
IC50 ) 7.4
R-mannosidase from almonds
NIb
99
IC50 ) 11.0
a Percentage of inhibition at 1 mM concentration, IC50 in µM, and optimal
pH.31 b NI ) no inhibition.
provided the hydrochloride (-)-7‚HCl in 79% yield (22%
overall yield based on lactol (-)-9; 41% based on nitrone (-)-
3). The selective reduction of lactam (-)-35 with BH3‚SMe2
in THF gave the â-hydroxyester (+)-38 in 81% yield. After
esterification of the alcohol (+)-38 with methanesulfonyl
chloride in CH2Cl2 and subsequent treatment with DBU at 20
°C, the R,â-unsaturated ester (+)-39 was isolated in 81% yield.
No alkene isomerization was observed under these conditions.
The reduction of ester (+)-39 with DIBAL-H ((i-Bu)2AlH) in
CH2Cl2, followed by a workup with HCl in MeOH, provided
7a-epi-crotanecine hydrochloride ((-)-8‚HCl) in 82% yield
(15% overall yield based on lactol (-)-9; 28% based on nitrone
(-)-3).
SCHEME 8
The new analogues of pyrrolizidine alkaloids (-)-7 and (-)-8
were tested for their inhibitory activities toward 22 commercially
available glycosidase enzymes.31 At 1 mM concentration,
compound (-)-7 showed no significant inhibition of the tested
glycosidases, apart from a weak inhibition of R-mannosidase
from jack beans (Table 1). Compound (-)-8 did not inhibit
R-fucosidases from bovine epididymis and human placenta,
R-galactosidase from E. coli, â-galactosidases from E. coli,
Aspergillus niger, and Aspergillus orizae, R-glucosidases from
yeast and rice, amyloglucosidase from Aspergillus niger,
â-glucosidase from caldocellum saccharolyticum, â-mannosi-
dase from Helix pomatia, â-xylosidase from Aspergillus niger,
R-N-acetylgalactosaminidase from chicken liver, or â-N-acetyl-
glucosaminidases from jack bean and bovine epididymis A and
B. For six other enzymes, the results are shown in Table 1.
as hyacinthacine A2.18,19b To illustrate further the usefulness of
cyclic nitrones, we have converted (-)-3 into rosmarinecine
and crotanecine analogues.
The reaction of (-)-3 with dimethyl maleate gave a 9.6:6:1
mixture of cycloadducts (-)-32, (+)-33, and (-)-34, which arise
from anti-exo, anti-endo, and syn-exo approaches, respectively
(Scheme 7). These products could be separated by column
chromatography on silica gel. In the case of the reaction of
racemic (()-3 with dimethyl maleate in benzene at room
temperature for 1 d, the minor adduct (()-34 had not been
detected.6a The structure of (-)-34 was deduced from its spectral
1
data. In particular, its H NMR spectrum showed at δH 3.97
Data show that (-)-8 is a selective inhibitor of R-mannosi-
dases. Fleet and co-workers had shown that pyrrolizidines 40
ppm a doublet of a doublet (J ) 5.4, 2.2 Hz), typical of the
bridgehead proton H-3a which is trans with respect to H-3 and
cis with respect to H-4. The main cycloadducts (-)-32 and (+)-
33 were converted into pyrrolizidinones (-)-35 and (+)-36,
respectively, upon hydrogenolysis in the presence of Pd(OH)2
catalyst. The reduction of (-)-32 and (+)-33 with Mo(CO)6/
H2O in acetonitrile25 gave less satisfactory results.6a
Pyrrolizidinone (-)-35 has been converted (Scheme 8) into
the new polyhydroxylated pyrrolizidines (-)-7 ((6R)-hydroxy-
7a-epi-rosmarinecine)26 and (-)-8.27-30 Treatment of (-)-35
with LiAlH4 in THF, followed by a workup with HCl in MeOH,
(27) For natural polyhydroxypyrrolizidines, see: (a) Asano, N.; Kuroi,
H.; Ikeda, K.; Kizu, H.; Kameda, Y.; Kato, A.; Adachi, I.; Watson, A. A.;
Nash, R. J.; Fleet, G. W. J. Tetrahedron: Asymmetry 2000, 11, 1-8. (b)
Atal, C. K.; Kapur, K. K.; Culvenor, C. C. J.; Smith, L. W. Tetrahedron
Lett. 1966, 537-544. (c) Mattocks, A. R. J. Chem. Soc. C 1968, 235-237.
(28) For the biological activity of crotanecine, see for example: (a)
Culvenor, C. C. J.; Edgar, J. A.; Jago, M. V.; Outteridge, A.; Peterson, J.
E.; Smith, L. W. Chem.-Biol. Interact. 1976, 12, 299-324. (b) Kato, A.;
Kano, E.; Adachi, I.; Molyneux, R. J.; Watson, A. A.; Nash, R. J.; Fleet,
G. W. J.; Wormald, M. R.; Kizu, H.; Ikeda, K.; Asano, N. Tetrahedron:
Asymmetry 2003, 14, 325-331. (c) Asano, N.; Ikeda, K.; Kasahara, M.;
Arai, Y.; Kizu, H. J. Nat. Prod. 2004, 67, 846-850.
(25) Cicchi, S.; Goti, A.; Guarna, A.; Brandi, A.; De Sarlo, F.
Tetrahedron Lett. 1990, 31, 3351-3354.
(26) For our previous synthesis of rosmarinecine using a malic acid
derived nitrone, see ref 6b.
(29) For the synthesis of crotanecine, see: (a) Yadav, V. K.; Rueger,
H.; Benn, M. Heterocycles 1984, 22, 2735-2738. (b) See ref 21. (c) Bennett,
R. B., III; Cha, J. K. Tetrahedron Lett. 1990, 31, 5337-5340. (d) Denmark,
S. E.; Thorarensen, A. J. Am. Chem. Soc. 1997, 119, 125-137.
J. Org. Chem, Vol. 71, No. 4, 2006 1617

Cicchi et al.
furanose (28),34 and 2,3,5-tri-O-benzyl-D-arabinofuranose (30),34
employed in this study were prepared according to published
procedures.
and 41 that are “contracted” swainsonine analogues are very
weak inhibitors of R-mannosidases (IC50 > 1000 µM).30a
Swainsonine (42) inhibits R-mannosidases from jack beans and
almonds with IC50 values of 0.2 and 0.4 µM, respectively.7a It
is, thus, a surprise that (-)-8 is only 1 order of magnitude less
active than swainsonine toward these enzymes, although the
structural changes between (-)-8 and 42 appear to be more
important than between 40 or 41 and 42. The much higher
inhibitory activity of (-)-8 toward R-mannosidases compared
with that of (-)-7 is noteworthy. It suggests that other
pyrrolidine and pyrrolizidine derivatives should be made because
they might have interesting inhibitory activities toward R-man-
nosidases and/or other glycosidases. Unfortunately, the glycosi-
dase inhibitory activities of crotanecine have not been reported
yet. From the data available, it seems that the alkene moiety at
C(2)-C(3) of (-)-8 and its (7aS) configuration are crucial to
render 1-hydroxymethylpyrrolizidines R-mannosidase inhibitors.
Obviously, the 6R,7S configuration of the diol moiety at C(6,7)
is also a requirement to make these systems capable of
recognizing R-mannosidases. As simpler 3,4-dihydroxypyrro-
lidines have been found to inhibit the growth of cancer cells,13c
derivatives of (-)-8 and analogues should be prepared and
assayed for their potential as antitumor agents.
General Procedure for the One-Pot Synthesis of Five-
Membered Cyclic Nitrones (-)-3 and (()-5. A 0.5 M solution
of lactol (-)-9 or (()-20 in dry pyridine (4 mL) was added with 3
Å molecular sieves (1.6 g) and hydroxylamine hydrochloride (1.2
equiv). The mixture was stirred overnight at room temperature, then
a 0.6 M solution of methanesulfonyl chloride (1.2 equiv) in dry
pyridine was added. The reaction mixture was stirred overnight at
room temperature and then diluted with dichloromethane (4 mL),
filtered through a Celite pad, concentrated, and purified by flash
column chromatography (FCC).
(3S,4R)-3,4-Isopropylidenedioxypyrroline 1-Oxide ((-)-3). A
54% yield was obtained; spectroscopic and analytical data were in
agreement with those previously reported.17
3-(tert-Butoxycarbonylamino)pyrroline N-Oxide ((()-5). A
colorless oil was obtained in a 36% yield (eluent, dichloromethane/
1
ethyl acetate/methanol, 15:7:1; Rf ) 0.13); H NMR δ 6.85 (q, J
) 1.8 Hz, 1H), 5.16 (m, 1H), 4.87 (br s, 1H), 4.71 (m, 1H), 3.92
(m, 1H), 2.70 (m, 1H), 2.02 (m, 1H), 1.44 (s, 9H); 13C NMR δ
154.9 (s), 135.0 (d), 80.5 (s), 61.5 (t), 52.5 (d), 28.4 (t), 28.4 (3C,
q). IR (CHCl3) ν˜max 3440, 3099, 1712, 1587, 1499, 1369 cm-1
.
MS (%) m/z 144 (58), 127 (100), 115 (12), 84 (69), 57 (97). Anal.
Calcd for C9H16N2O3: C, 53.99; H, 8.05; N, 13.99. Found: C, 53.68;
H, 8.40; N, 14.15.
Conclusions
General Procedure for the Synthesis of Nitrones (-)-4 and
(-)-6. A 0.5 M solution of lactol 28 or 30 in dry pyridine (2 mL)
was added with 3 Å molecular sieves (800 mg) and hydroxylamine
hydrochloride (1.2 equiv). The mixture was stirred overnight at
room temperature, then a 0.6 M solution of methanesulfonyl
chloride (1.2 equiv) in pyridine was added. The reaction mixture
was stirred overnight at room temperature and then filtered through
Celite. The filter was washed with dioxane (10 mL), and the
collected solution was cooled at 0 °C. A cooled 2 M NaOH solution
was added dropwise until pH ) 10. The mixture was stirred at 0
°C for 2 h, maintaining the solution at pH > 9, then dioxane was
removed in vacuo without heating. The solution was adjusted to
pH ) 7 by the dropwise addition of a cooled 2 M HCl solution.
The resulting mixture was extracted with dichloromethane (3 ×
50 mL). The collected organic phase was dried with Na2SO4,
filtered, concentrated, and purified by FCC.
In conclusion, we have developed and studied the scope of a
novel variant for the formation of pyrroline N-oxides by
intramolecular nucleophilic displacement, which is based on a
simple one-pot procedure employing inexpensive hydroxy-
lamine, methanesulfonyl chloride, and lactols. One cyclic nitrone
obtained in this work has been used to prepare two new
polyhydroxylated pyrrolizidines. One of them, (-)-8, is a good,
selective inhibitor of R-mannosidases.
Experimental Section
The noncommercially available lactols, (-)-9,32 2-(tert-butoxy-
carbonylamino)-γ-butyrolactol (20),22,33 2,3,5-tri-O-benzyl-L-xylo-
Nitrone (-)-4.18,19 A white solid was obtained in a 25% yield
(eluent, dichloromethane/ethyl acetate, 20:7; Rf ) 0.40). Mp 92-
93 °C. [R]19 -41.7 (c 1.00, CHCl3) [lit.19a mp 88-90 °C, [R]20
(30) For the synthesis of other polyhydroxypyrrolizidines, see for
example: (a) Carpenter, N. M.; Fleet, G. W. J.; Cenci di Bello, I.;
Winchester, B.; Fellows, L. E.; Nash, R. J. Tetrahedron Lett. 1989, 30,
7261-7264. (b) Burgess, K.; Henderson, I. Tetrahedron Lett. 1990, 31,
6949-6952. (c) Hudlicky, T.; Luna, H.; Price, J. D.; Rulin, F. J. Org. Chem.
1990, 55, 4683-4687. (d) Hudlicky, T.; Fan, R.; Luna, H.; Olivo, H.; Price,
J. Indian J. Chem., Sect. B 1993, 32, 154-158. (e) Casiraghi, G.; Spanu,
P.; Rassu, G.; Pinna, L.; Ulgheri, F. J. Org. Chem. 1994, 59, 2906-2909.
(f) Bell, A. A.; Pickering, L.; Watson, A. A.; Nash, R. J.; Pan, Y. T.; Elbein,
A. D.; Fleet, G. W. J. Tetrahedron Lett. 1997, 38, 5869-5872. (g) De
Vicente, J.; Arrayas, R. G.; Carretero, J. C. Tetrahedron Lett. 1999, 40,
6083-6086. (h) Pearson, W. H.; Hines, J. V. J. Org. Chem. 2000, 65, 5785-
5793. (i) Romero, A.; Wong, C.-H. J. Org. Chem. 2000, 65, 8264-8268.
(j) White, J. D.; Hrnciar, P. J. Org. Chem. 2000, 65, 9129-9142. (k) Ahn,
J.-B.; Yun, C.-S.; Kim, K. H.; Ha, D.-C. J. Org. Chem. 2000, 65, 9249-
9251. (l) Denmark, S. E.; Cottell, J. J. J. Org. Chem. 2001, 66, 4276-
4284 and references therein. (m) Rambaud, L.; Compain, P.; Martin, O. R.
Tetrahedron: Asymmetry 2001, 12, 1807-1809. (n) Kuban, J.; Kolarovic,
A.; Fisera, L.; Jaeger, V.; Humpa, O.; Pronayova, N. Synlett 2001, 1866-
1868. (o) Lindsay, K. B.; Tang, M.; Pyne, S. G. Synlett 2002, 731-734.
(p) Behr, J.-B.; Erard, A.; Guillerm, G. Eur. J. Org. Chem. 2002, 1256-
1262. (q) Carmona, A. T.; Fuentes, J.; Vogel, P.; Robina, I. Tetrahedron:
Asymmetry 2004, 15, 323-333. (r) Izquierdo, I.; Plaza, M. T.; Tamayo, J.
A. Tetrahedron: Asymmetry 2004, 15, 3635-3642.
D
D
-41.7 (c 1.00, CHCl3); lit.19b mp 91-93 °C, [R]20 -42 (c 1.3,
D
1
CHCl3)]. H NMR δ 7.38-7.26 (m, 15H), 6.91 (d, J ) 1.9 Hz,
1H), 4.69-4.67 (m, 1H), 4.64-4.46 (m, 6H), 4.39 (dd, J ) 3.2,
2.2 Hz, 1H), 4.10-4.04 (m, 2H), 3.78 (d, J ) 7.3 Hz, 1H). 13C
NMR δ 137.7 (s), 137.2 (s), 137.1 (s), 132.9 (d), 128.6-127.7 (d,
15C), 82.7 (d), 80.3 (d), 77.4 (d), 73.9 (t), 71.9 (t), 71.6 (t), 66.1
(t). IR (CHCl3) ν˜max 3040, 2990, 2910, 1455, 1380, 1218 cm-1
.
MS (%) m/z 285 (6), 234 (3), 132 (19), 91 (100), 77 (28). Anal.
Calcd for C26H27NO4: C, 74.80; H, 6.52; N, 3.35. Found: C, 74.51;
H, 6.78; N, 3.01.
Nitrone (-)-6. A dark yellow oil was obtained in a 33% yield;
Rf ) 0.38 (dichloromethane/ethyl acetate, 20:7). [R]23 -75.9 (c
D
0.54, CH2Cl2). 1H NMR δ 7.40-7.20 (m, 15H), 6.80 (s, 1H), 4.72
(d, J ) 2.9 Hz, 1H), 4.69-4.47 (m, 6H), 4.36 (dd, J ) 7.7, 4.3
Hz, 1H), 4.16 (m, 1H), 3.98 (dd, J ) 10.0, 4.3 Hz, 1H), 3.81 (dd,
J ) 10.0, 2.0 Hz, 1H).13C NMR δ 137.8 (s), 137.2 (s), 137.1 (s),
133.7 (d), 128.5-127.5 (d, 15C), 83.1 (d), 80.5 (d), 74.0 (t), 73.5
(t), 73.1 (t), 72.4 (t), 64.3 (d). IR (CHCl3) ν˜max 3033, 2924, 2870,
1582, 1454, 1099 cm-1. MS (%) m/z 326 (11), 218 (8), 181 (18),
(31) Brandi, A.; Cicchi, S.; Cordero, F. M.; Frignoli, R.; Goti, A.; Picasso,
S.; Vogel, P. J. Org. Chem. 1995, 60, 6806-6812.
(32) Thompson, D. K.; Hubert, C. N.; Wightman, R. H. Tetrahedron
1993, 49, 3827-3840.
(33) Tatsuta, K.; Miura, S.; Ohta, S.; Gunji, H. Tetrahedron Lett. 1995,
36, 1085-1088.
(34) Tejima, S.; Fletcher, H. G., Jr. J. Org. Chem. 1963, 28, 2999-
3004.
1618 J. Org. Chem., Vol. 71, No. 4, 2006

One-Pot Synthesis of Cyclic Nitrones
91 (100), 77 (20), 65 (27). FAB + 418. Anal. Calcd for C26H27-
NO4: C, 74.80; H, 6.52; N, 3.35. Found: C, 74.91; H, 6.65; N,
3.57.
and potassium tartrate was added (1 mL). The mixture was stirred
overnight and then quickly passed through a short pad of silica
gel. The solution was concentrated to give a solid residue that was
directly dissolved in MeOH (5 mL). The resulting solution was
treated at 0 °C with a 1 M HCl solution in MeOH (2 mL), stirred
for 1 h at room temperature, and then concentrated in vacuo to
give a pale yellow oil, which was purified by crystallization from
ethanol/ether to afford (-)-8‚HCl (85 mg, 82%) as an extremely
hygroscopic white solid. [R]23D -17.8 (c 0.04, CH3OH). 1H NMR
(D2O) δ 5.61 (br s, 1H), 4.87 (m, 1H), 4.32-4.22 (m, 3H), 4.17
(m, 2H), 3.82 (br d, J ) 15.3 Hz, 1H), 3.68 (dd, J ) 12.5, 2.2 Hz,
1H), 3.22 (dd, J ) 12.5, 2.9 Hz, 1H). 13C NMR (D2O) δ 138.2 (s),
119.2 (d), 76.1 (d), 73.8 (d), 70.3 (d), 60.3 (t), 59.0 (t), 56.8 (t).
MS (%) m/z 207 (M+, 2), 172 (19), 112 (93), 94 (62), 82 (100), 80
(94), 69 (74). Anal. Calcd for C8H14NO3Cl‚H2O: C, 42.58; H, 7.15;
N, 6.21. Found: C, 42.34; H, 6.94; N, 6.54.
(6R)-Hydroxy-7a-epi-rosmarinecine Hydrochloride ((-)-7‚
HCl). A 1 M solution of LiAlH4 in THF (1.5 mL) was slowly added
to a solution of pyrrolizidinone (-)-35 (135 mg, 0.5 mmol) in THF
(5 mL) cooled to 0 °C. The mixture was heated at reflux for 1.5 h
and then cooled to 0 °C. A saturated aq Na2SO4 solution was added
to the stirred mixture. After filtration through Celite and washing
with ethyl acetate, the solution was concentrated to give a solid
residue that was directly dissolved in MeOH (5 mL). The resulting
solution was treated at 0 °C with a 1 M HCl solution in MeOH (4
mL), stirred for 1 h at room temperature, and then concentrated in
vacuo to give a pale yellow oil that was purified by crystallization
from ethanol/ether to afford (-)-7‚HCl as a white solid (75 mg,
79%). Mp 158-159 °C. [R]24D -23.1 (c 0.27, CH3OH). 1H NMR
(D2O, 400 MHz) δ 4.49-4.46 (m, 1H), 4.46-4.44 (m, 2H), 3.85
(dd, J ) 7.0, 4.4 Hz, 1H), 3.72 (dd, J ) 12.6, 2.2 Hz, 1H), 3.66
(dd, J ) 13.0, 5.0 Hz, 1H), 3.63-3.56 (m, 2H), 3.50 (dd, J )
12.6, 3.3 Hz, 1H), 3.25 (dd, J ) 13.0, 3.1 Hz, 1H), 2.51-2.46 (m,
1H). 13C NMR (D2O, 100 MHz) δ 76.3 (d), 72.8 (d), 70.4 (d),
69.8 (d), 59.4 (t), 59.3 (t), 58.5 (t), 50.4 (d). IR (KBr) ν˜max 3483-
3227 (br) cm-1. MS (%) m/z 189 (1), 188 (2), 171 (9), 98 (50), 97
(100). Anal. Calcd for C8H16NO4Cl: C, 42.58; H, 7.15; N, 6.21.
Found: C, 42.49; H, 7.41; N, 5.96.
Acknowledgment. We thank MIUR - Italy (PRIN 2002)
and the Swiss National Science Foundation for financial support
and Ente Cassa di Risparmio di Firenze, Italy, for granting a
400 MHz NMR spectrometer. Technical assistance from
Brunella Innocenti, Maurizio Passaponti, and Catherine Schuetz
is gratefully acknowledged.
Supporting Information Available: Experimental details,
synthetic procedures, and characterization data for all newly
synthesized compounds and 1H and 13C NMR spectra of compounds
5-8. This material is available free of charge via the Internet at
http://pubs.acs.org.
7a-epi-Crotanecine Hydrochloride ((-)-8‚HCl). A 1 M solu-
tion of DIBAL-H (3.3 equiv) in dichloromethane was slowly added
to a solution of (+)-39 (120 mg, 0.5 mmol) in dichloromethane (5
mL) cooled to 0 °C. The reaction was stirred at 0 °C for 30 min
and then quenched with methanol (2 mL). The suspension was
diluted with CH2Cl2 (5 mL), and a saturated aq solution of sodium
JO0523518
J. Org. Chem, Vol. 71, No. 4, 2006 1619